Technical Field and Prior Art
The present invention relates to the field of measurement tools, e.g., suitable for use in equipment for oil prospecting and production.
More specifically, after a well has been bored, that type of activity requires sondes or sensors, in particular electrical or electromagnetic sondes or sensors to be inserted into the hole to enable measurements to be performed serving to characterize, amongst others, which fluids are present in the terrain and layers around the borehole, and also the dip of said layers. The term "logging" is used to designate any continuous recording as a function of depth of variations in a given characteristic of the formations around a borehole.
One of the characteristics that it is important to know in a borehole is the resistivity of the drilling mud used. The resistivity of the mud is a parameter that is used, in particular, to correct measurements relating to other characteristics of the surrounding formations. In order to discover this mud resistivity, various approaches are already known.
In a first approach, mud resistivity is measured by a device that requires additional equipment on the tool already used for measuring the characteristics of the formation, which additional equipment may be, for example, of the AMS type (described in document EP-013 224). That technique gives rise to additional costs and to apparatus that is of greater bulk.
In another technique, the resistivity of the mud is measured at the surface from a fluid sample. Extrapolation then makes it possible to take account of temperature dependence relative to downhole conditions by measuring the temperature down hole. The accuracy obtained is often unsatisfactory, essentially for the following two reasons:
difficultly in obtaining an accurate measurement of the temperature downhole; and PA1 the characteristics of the fluid in the borehole can change with depth, in which case the sample available on the surface is no longer representative. PA1 inserting a sonde into the borehole, the sonde having an elongate body provided with at least one annular current electrode and at least two annular guard electrodes situated on either side of the annular current electrode; PA1 emitting at least one current I.sub.0 into the surrounding formation from the annular current electrode; PA1 focusing the current I.sub.0 in the formation by emitting two currents I.sub.1 and I'.sub.1 from the annular guard electrodes situated on either side of the annular current electrode; and PA1 producing a signal in response to the emitted current I.sub.0, which signal is representative of the resistivity R.sub.m of the drilling mud. PA1 two annular current electrodes respectively emitting a current I.sub.0 and a current I'.sub.0 into the surrounding formation; and PA1 an annular potential-measuring electrode situated between the two current electrodes or else an array of azimuth electrodes situated between the two annular current electrodes. PA1 a sonde having an elongate body provided with at least one annular current electrode and at least two annular guard electrodes situated on either side of the annular current electrode; PA1 means for emitting at least one current I.sub.0 into the surrounding formation from the annular current electrode; PA1 means for focusing the current I.sub.0 in the formation by emitting two currents I.sub.1 and I'1 from the two annular guard electrodes situated on either side of the annular current electrode; and PA1 means for producing a signal in response to the emission of the current I.sub.0, said signal being representative of the resistivity R.sub.m of the drilling mud. PA1 two annular current electrodes; PA1 means for emitting into a surrounding formation a current I.sub.0 via one of the annular electrodes, and a current I'.sub.0 via the other annular electrode; PA1 an annular electrode for measuring potential, situated between the two current electrodes, or else an array of azimuth electrodes situated between the two annular current electrodes. PA1 inserting a sonde into the borehole, the sonde having an elongate body provided with at least one annular current electrode and at least two annular guard electrodes situated on either side of the annular current electrode; PA1 performing computed focusing to simulate an operating mode in which: PA1 producing a signal representative of the resistivity R.sub.m of the drilling mud from the simulated operating mode. PA1 a first mode in which current having great penetration depth is emitted into the surrounding formations; and PA1 a second mode in which current having shallow penetration depth is emitted into the surrounding formations. PA1 a first operating mode in which current is emitted into the surrounding formation, specifically a current i.sub.1 from one of the annular guard electrodes and a current i'.sub.1 from the other annular guard electrode, the current emitted by the annular current electrode(s) being equal to 0; PA1 a second operating mode in which at least one current i.sub.0 is emitted from the annular current electrode(s) to the annular guard electrodes, with the total current emitted from the sonde into the formation being equal to 0. PA1 firstly the focusing voltage and the sonde voltage; and PA1 secondly the current emitted from the current electrode(s) and the total current emitted into the formation. PA1 a single current electrode; PA1 first, second, and third pairs of potential-measuring electrodes disposed on either side of the current electrode; PA1 the focusing voltage being equal to the difference V.sub.1 -V.sub.2 between the mean voltages from the first and second pairs of potential-measuring electrodes; PA1 the sonde voltage being equal to the difference V.sub.2 -V.sub.3 between the mean voltages from the second and third pairs of potential-measuring electrodes. PA1 two annular current electrodes; PA1 and: PA1 and first and second pairs of annular potential-measuring electrodes; PA1 the focusing voltage being equal to the difference between the mean voltage of the first pair of annular potential-measuring electrodes and either the voltage of the annular potential electrode disposed between the two current electrodes, or the mean voltage of the array of azimuth electrodes; PA1 the sonde voltage being equal to the difference between the mean voltages of the first and second pairs of annular potential-measuring electrodes. PA1 a sonde having an elongate body provided with at least one annular current electrode and at least two annular guard electrodes situated on either side of the annular current electrode; PA1 means for performing computed focusing so as to simulate an operating mode in which: PA1 means for computing a signal representative of the resistivity R.sub.m of the drilling mud on the basis of the simulated operating mode. PA1 means for emitting into the surrounding formation in a first effective operating mode both a current i.sub.1 from one of the annular guard electrodes and a current i'.sub.1 from the other annular guard electrode, the current emitted from the annular current electrode(s) being equal to 0; PA1 means for emitting, in a second effective operating mode, at least one current i.sub.0 from the annular current electrode(s) to the annular guard electrodes, the total current emitted from the sonde into the formation being equal to 0; PA1 the means for performing computed focusing operating on the basis of these two effective operating modes. PA1 signals representative of a focusing voltage and of a sonde voltage; PA1 a signal representative of the current(s) emitted from the current electrode(s). PA1 means being provided for producing in the first effective operating mode, a signal representative of the total current emitted into the formation; PA1 the means for performing computed focusing enabling transfer impedances or coefficients to be deduced between: PA1 the highly resistive nature of the surrounding formation; PA1 the presence of one or more highly conductive beds in the formation; PA1 the influence of the borehole.